1. Field of the Invention
The present invention relates to a method and apparatus for heat treating a particulate product, such as drying and/or cooling such product.
2. Description of Prior Art
It is well-known to dry a moist, particulate product in a fluid bed dryer. This moist product may, for example, have been prepared from a solution or slurry in a spray dryer. A single stage spray drying process may be replaced by a two-stage process, i.e. spray drying followed by fluid bed drying, in order to obtain a capacity increase and/or a better product quality. In this two-stage drying process it is often desirable to transfer the moist, particulate product prepared in the spray dryer to the fluid bed dryer with the highest possible residual moisture content. Furthermore, some products tend to form lumps during spray drying or other manufacturing processes. The presence of lumps may present problems for a subsequent fluid bed drying of such products. In some cases, it is possible to dry a particulate product which is not immediately fluidizable, if the product comprising small lumps can be fed to the top of a totally mixed, so-called back mixed fluidized layer, for example by means of a rotating product spreader. Such a technique is known from U.S. Pat. No. 3,771,237, which discloses a two-stage fluid bed. The first stage is a back mixed bed receiving the wet feed, and the second stage is a plug flow fluid bed for drying the product to its final residual moisture content.
However, if a product contains big lumps, it is not possible to achieve a uniform drying in a back mixed bed, and drying in a plug flow bed is also out of question.
U.S. Pat. No. 4,787,152 discloses a fluid bed dryer by means of which it is possible to treat products having a wide range of particle sizes. This known fluid bed dryer is provided with a product feeder or delumper comprising alternate rows of cooperating discs carried by two parallel shafts and arranged in a housing into which hot drying air is fed.
U.S. Pat. No. 3,370,938 discloses a method and apparatus particularly for roasting finely divided particles of metal sulfide ores. Agglomerated bed particles are fluidized in a second fluid bed, which is positioned below and opens into the bed plate of a first main fluid bed. The agglomerated, relatively coarse bed particles are currently removed from the second fluid bed, while relatively fine bed particles are removed from the first fluid bed suspended in the fluidizing gas discharged therefrom. The fine particles may be returned to the fluidized bed system.
DE 2929056 A1 discloses a fluid bed used in a quite different field, namely in a combustion chamber. This fluid bed is formed by inert, non-combustible fluidizable particles above a perforated bed plate. Non-fluidizable particles of foreign matter which have inadvertently been introduced into the fluid bed are directed to discharge passages by the fluidizing air flows which are passed upwardly through the bed plate.